Microsoft's Internet Explorer (IE) again lost a big chunk of market share last month as Mozilla's Firefox and Google's Chrome continued to chip away at the world's dominant browser, a Web metrics company said today.

IE plummeted by 1.3 percentage points in September, falling to 65.7%, it's lowest level ever recorded and the largest one-month drop in almost a year, said Net Applications.

In the last 12 months, IE has lost 8.5 percentage points and has dropped 2.6 points in the last three months alone. Unless Microsoft is able to stem the slide, its browser will slip under the 50% mark in the next two years.

As has been the trend of late, IE's pain has been Firefox's and Chrome's gain.

Mozilla's browser surged by 0.8 of a percentage point, the biggest single-month gain since November 2008, to account for 23.8% of the browser market. Firefox has still not recovered, however, from a share downturn during May and June 2009, and has yet to equal the record set in April of 23.84%. It should pass that mark next month if it continues on the pace it's set in the last 12 months.

Chrome, meanwhile, posted another record gain during September, increasing its share by 0.3 of a percentage point to end the month at 3.2%. In the last three months, Chrome's share has increased by nearly 0.8 of a percentage point, more than half the gains by Mozilla during the same period.

Safari, however, seems stalled, boosting its market share slightly to 4.2%. Apple's browser has gained just 0.45 of a percentage point in the last three months, about one-third as much as Firefox has grown in the same span and a little more than half that of Chrome's increase.

With the vast bulk of Safari's users on Macs -- Safari for Windows accounted for less than a third of a percentage point last month -- Apple's browser gains are directly linked to Mac sales.

Opera Software's Opera accounted for 2.2%, growing by 0.15 of a percentage point, the Norwegian browser's largest single-month gain since January 2008.

Both Google and Opera shipped new versions of their browsers last month. Google rolled out Chrome 3.0 on Sept. 15, bragging up a 25% speed increase over the previous edition, while Opera 10 debuted Sept. 1.

Microsoft and Mozilla, meanwhile, managed to move even more of their users to their newest browsers.

The eight-year-old IE6 lost 0.8 of a percentage point, dropping to 24.4%, while 2007's IE7 lost 1.7 points, falling to 19.4%. IE8 gained 1.7 percentage points to post a September average of 16.8%, another record for it. As recently as April, IE8 accounted for only 3.6% of all browsers.

Microsoft's campaign to get users off its eldest browser, IE6, however, seems to be mired. While the aged application lost nearly 4 percentage points in July and August, its September slide was under a point.

IE6, said Microsoft's manager of Internet Explorer in August, should beabandoned by users able to pick their browser. IE6, however, still accounted for 38% of IE's worldwide total last month, and remains the most popular Microsoft browser by a substantial margin.

But IE8 has been making progress, particularly against its immediate predecessor. The former's 16.8% share is within striking distance of IE7's 19.4%; if current trends persist, IE8 could pass IE7 in share as early as this month.

Mozilla has also made impressive strides in migrating its users to Firefox 3.5, the upgrade launched last June. In September, Firefox increased its share by 3.8 percentage points to 12.7%, while Firefox 3.0 slipped 2.9 points to end at 9.6%. September was the first month that Firefox 3.5 accounted for a greater percentage of all Firefox users than the older Firefox 3.0. The first automatic update offer from Firefox 3.0 to 3.5 in mid-August may have sparked the surge in the latter's share.

Factors that may affect future share rankings include Google's recent release of Chrome Frame, a plug-in for IE that lets Web sites trigger the use of Chrome's WebKit rendering engine and its V8 JavaScript engine. When IE uses Chrome Frame, sites identify it as IE, but add a Chrome Frame tag that could let analytics firms like Net Applications spot the use of the Chrome rendering engine.

It's unknown whether Net Applications will actually do that, however; the company did not respond to a request for comment.

Net Applications measures browser usage by tracking the machines that surf to the 40,000 sites it monitors for clients, which results in a data pool of about 160 million unique visitors per month.